I cut the top off a postmix keg and attached it to a regular 50L keg, it works great.. have 2 of them now.
The total cost for making this up was $140 per keg, they are great, only trouble is they are heavy, very heavy! (lucky I have a forklift :D )
I put my kegs in the fridge and connect the gas at serving pressure (~80kpa) and let them sit in there for anywhere between a couple of weeks and a couple of months.
It gives them time to carbonate slowly and age/cold condition.
I split a double batch between two kegs and shook the first keg...
Since I have moved in to a 2 bedroom unit there is no beer garden here, just a small deck that just fits a small BBQ and a couple of chairs.. still it's good to go out there for a beer or two in the afternoon, not much to look at though.
Grolsch is one of my favorite beers and being a K&K person the brewcraft dutch lager with extra hallertau hops in it comes very close to it after 6 weeks of fermenting/racking and then 3 months CC'ing.
It takes a long time, but very well worth it, got 50Ltrs of it ready to go soon and I can't wait!